Short Wave - Micro Wave: Build Your Own Sandcastle Dreamhouse

Episode Date: August 20, 2021

It's summer, which for some means spare time at the beach, splashing in the waves and...building sandcastles. On today's episode, Emily Kwong asks: Scientifically, what is the best way to make a sandcastle? What's the right mix of water and sand to create grand staircases and towers? Sedimentologist Matthew Bennett shares his research — and personal — insights. Happy building! Wondering what science and engineering are behind other summertime activities? Or just want to share your greatest sandcastle creations? Shoot us an email at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

So Maddie. Yeah, bud. I think it's time to tell the good people of Shortwave that your last two weeks as host are upon us. It's true. It's true. But nobody cry. Nobody cry yet. Don't you dare.

Don't you dare. You will be packing this time with as much science, shortwave memories, and just, you know, just general nerd clowning as we can. That is right. It's going to be magical. And today we've got a microwave for you, those little shorty episodes with a fun science fact and some listener mail. And Emily, in my notes, it says you're taking me to a beach right now. Yeah, dude.

The world's been, I don't know, a little stressful lately. Yeah. So, Rebecca Ramirez, almighty producer, cue the waves. It's just like instantly relaxing, you know? Makes me think of hermit crabs, you know, my complicated relationship with sand, like get out of my nucks and crannies rock dust. You know what I'm saying? It's true.

I mean, I remember every summer going up. up to Maine with my family and building sandcastles, okay? And not like flip a bucket and you're done kind of sandcastles. I'm talking towers, staircases, moats, seaweed accents. Ooh, little accent, little accent. Would you ever do that thing where you add a ton of water to the sand? You get these like cool drizzles that kind of look like stalactites? Oh yeah, you mean art?

Sure. Yeah, that's what I mean art. I'm kind of glad you brought this up actually because, you know, when it comes to sand castle building, how much water you add is actually really important. Add too much water and it gets mushy. Add too little, the castle crumbles. Sedimentologist Matthew Bennett has spent a lot of time thinking about why and looking at sand under a microscope. Particularly when you've got the light glistening through it, it looks like a set of gemstones.

It's really beautiful. And you can say an awful lot about its history, it's transport history. Has it been transported by wind, water, or ice? And that's what I find exciting to read its story. Matthew is a professor of environmental and geographical science at Bournemouth University. A few years ago, he wrote an article called How to Build the Perfect Sandcastle according to science. I reached him while he was on vacation in Wales. Do you have any sandcastle building plans?

I always have sandcastle building plans. There's nothing nicer than a good sandy beach and a sandcastle building competition with the children. So today on the show, grab your buckets and shovels, we are going to build a sand castle and talk about why water plays such a key role in their construction. This is Shortwave, the Daily Science podcast from NPR. So today we're talking about the science that holds a sandcastle together and maybe some tips on how to build a sandcastle. If you're lucky. Okay, first let's talk about sand.

Yes. So sand is the crushed up remains of rocks. Any rocks. Rocks that have been worked backwards and forwards and crushed by natural processes ground down to individual mineral grains. And a lot of sand has this particular. mineral in it known as quartz. Quartz crystals are some of the most resistant to weathering and erosion, natural processes. And so a lot of sand is dominated by quartz crystals.

And when you look at sand under a microscope, you can see those natural processes that shaped it and where it came from. So if it's a sand that comes out of a sand dune, for example, then all the sand grains have the frosted, sort of sandblasted look. If it comes out of a very glacial environment, they're very sharp, that they're angular and quite fresh. Rivers and beach sands tend to be still very rounded, but a lot less frosted. And they're also not as red, because the desert sands, it's the iron, the iron oxide

in it that makes it very red. That's why desert sands are always so red. and beach sands are a much sort of paler color. And speaking of beach sands, this one's for you, Maddie. The white sand beaches of Hawaii are due in part to parrot fish poop. Poop back, poop back. Okay, so the important thing to remember is that sand comes from everywhere and pure sand, which is just crushed rock, doesn't stick together.

You need water to provide that glue. So here, I want you to do an experiment with me. All right, get a glass of water. Right, like right now? Yeah. I'm in a closet. Just get the water. Don't you have a water bottle?

Yeah, I know I've got it. Not in the computer. I want you to dip your thumb and point your finger in the water. Gross, okay. All right. You can get out different water later. Okay. Yeah.

All right. Touch the tips of them together. Uh-huh. And now gently, slowly, pull them apart and tell me what you see. Oh, like a little water, little water bridge. Yeah. Kind of stretch. And then breaks.

Okay, that water bridge is called a capillary bridge. The water molecules are more attracted to each other than they are to your fingers, forming those bridges. And in sandcastles, this is what holds sand grains together. Okay, so sandcastle's really about us taking advantage of this cohesion property of water and turning it into art. Well, yeah, like all art, though, it's all about restraint. If you've got too much water, then what happens? is that it becomes lubricant and the grains can slide and move past one another. If you've not got enough water, then there's not in this suction taking place.

So just getting the critical right amount of water is what's important. And that's the work, really, that holding and binding that sand together. Nice. Okay. Do certain kinds of grains lend themselves to like a stronger sand castle than others? Yeah, he thinks so. So Matthew a few years ago surveyed 10 beaches, around the UK. This wasn't for science. It was for a magazine that wanted to know the best sandcastle beach. Truly a dream research question. Go on. Yeah. So Matthew and his colleagues found that in beaches where the grains were slightly more angular, they locked together better. And the

finer the grains, so the smaller the sand grains are, the better they lock together too. Why is that? Well, fine sand. And this is backed by every professional sandcastle builder I've read about. They all say that fine sand is better at holding water, meaning they form even better capillary bridges because they have even more pores that can hold water. Got it. You need some good sand. And you need a good spot, right? Not too near the waves, but not too high up the beach. So you don't have to like haul buckets of water to get the sand wet enough. I like that we've gotten into logistics. I like to pick a spot that's like below the tide line where the sand starts to just change color. Like, it's just wet enough to stick together. Yeah, that's a good, like, site test.

And you can do a texture test, too. So Justin Gordon, he's a sculptor in Massachusetts. He says if you squeeze a ball of sand in your palm and it holds when you roll it around your palm, you're in business. Got it. Then begins construction. So my uncle would usually build a mound of sand and pour water on top, really packing the sand down with his hands and tools. And you'll notice the sand, it gets really. hard. That's the capillary action kicking in. Strong Castle needs a strong foundation. As in life.

As in life. But Emily, like, you've convinced me that water is really important. Is there like a right amount of water to add? Has science answered this question? No. Scientists have done experiments, kind of, like even one in a laboratory settings I was able to read about. But honestly, it depends on the sand, right? So when you go to the beach, just use your senses. See and feel how the sand is holding together. Remember, it's a discovery process, and it actually has real applications in civil engineering. Builders, they have to figure out the stability of the soil and see whether it can hold up to stress from the elements. And that's something you can test out on your sandcastle too.

If you build your sandcastle and then thomp with your hand or the flat of your spade, the beach right next to your sandcastle, It's a bit like your own mini earthquake and it will destabilize the walls of your sandcastle. Yeah, Emily, I have definitely, you know, smash somebody else's sandcastle in the name of science, not jealousy. There's some kids out there listening who know what you mean. Okay, Emily Kwong, thank you for the sandcastle science. Are you ready for some listener mail? Let's do it. All right.

So a lot of folks responded to that story that you reported a while back about learning Mandarin Chinese as an adult for the series. where we come from. It was all about how losing a language impacted your family. And I want to read you some of those notes because people really identified with this episode. Okay. Yes. Big breaths. Big breaths. Big breaths. I'm excited to hear these. All right. Dear Emily, your story really struck a chord with me as I am a first generation immigrant from Hong Kong and I speak Cantonese. Your podcast has renewed my resolve to teach my kids more Cantonese. It made me emotional, listening to it, probably because I didn't realize how conflicted I felt about this topic. It was like a dam had opened up. However, at the same time, I feel like somebody finally understands

and that I'm not alone. Thank you for doing such nuanced coverage on this topic. Sincerely, Josephine Lee, in Texas. Yeah, Josephine, I'm not going to lie. I was super nervous about this story because it is so personal. And like the fact that it validated something for you, opened up a conversation for you, that's like the best outcome a story could have. So thank you so much for writing that. Classic Emily Kwong stuff.

Classic Emily Kwong. Okay, hold on to your butt. Here comes another one. Cool. Okay. And this one is from Kristen in California. Like the other one, I'm just going to read parts of it. This episode gave us listeners the privilege

of hearing Emily with our hearts as well as our mind. For all of us, it allowed us to connect more personally with Emily herself so that the next time we hear her friendly voice and fabulous laugh on shortwave will be hearing a closer friend than we heard before. Thank you for all these gifts. Kristen, that is a beautiful email. I have so much gratitude for being able to work on this story and share it with you and all of you. And honestly, to the shortwave team, because they let me work on it on the side, right? It wasn't a science story, and yet I think what's cool about our show is we really try to, like, see people as their whole selves. And this is a big part of me.

So thanks for listening to it. Well, thank you, Emily Kwong, for sharing yourself. What a beautiful self to share. What a beautiful self to share. Well, all right, Maddie, yep. Back next week. Back next week. Yeah, yeah.

This episode was produced by Rebecca Ramirez, edited by Viet Le and fact-checked by Indy Kara. The audio engineers were Gilly Moon and James Willits. Thanks for listening to Shortwave, the Daily Science Podcast from NPR.